CEA DSM Irfu Reconstruction and analysis of ANTARES 5 line data Niccolò Cottini on behalf of the ANTARES Collaboration XX th Rencontres de Blois 21 / 05

CEA DSM Irfu

Reconstruction and analysis of ANTARES 5 line data

Niccolò Cottini

on behalf of the ANTARES Collaboration

XXth Rencontres de Blois

21 / 05 / 2008

21 / 05 / 2008 Niccolò Cottini 2CEA DSM Irfu

Outline of the talk

• The ANTARES experiment– Detector layout– Data acquisition system

• Data analysis– Event reconstruction– Selection of neutrino events

• standard approach• an example of the work being pursued


The ANTARES detector

12 lines, 900 PMTs-2500 m

450 m

~60 m


The detected signal

• Muon (Cherenkov)– 2 µs crossing time

• 40K decay– Continuous background

30 kHz *

• Bioluminescence– Continuous background

30 kHz *– Occasional bursts ~MHz

Simulation

2 min

* PMT 10’’, threshold 0,3 p.e.


The Trigger

• Front end chip digitizes charge and time of a light signal

“ALL DATA TO SHORE” SCHEME:

• all data transmitted through multiplexed Gigabit links

• computer farm running a software trigger:

• the whole data flow can not be written to disk

• look in all directions for light signals compatible with a muon track

• muon triggered event rate: ~1 Hz


The physics signal

Keep all the muon tracks;

then filter to extract the neutrino signal

Muon flux at the detector

Atmosphere

Earth

Sea

107 atmospheric per year3000 atmospheric

per year

Cosmic


5 line data analyzed

169 days

29 Jan 2007 4 Dec 2007310 days


43°

interaction

Sea floor

Cherenkov light from

3D PMTarray

Event reconstruction

Reconstruction of trajectoryfrom time, charge and position of PMT hits



z-t event display

z

y

x



Complicated pattern; a bundle can be misreconstructed as an upward going track

Atmospheric muon bundle


Reconstruction algorithm

• Muon track fit in 3 steps:

1) Prefit: (x,y,z,t) linear fit through the photon hits

2) Improved 2 minimization (charge weights)

3) Time residual pdf (charge weights)

Direct Cherenkov emission

Diffused light

Electronics effect

Local minima possible: reiteration with 8 different starting points (prefit rotations)


Reconstruction quality (MC simulation with 12 lines detector)


5 line data reconstruction

Dominated by the tail of atmospheric muons badly reconstructed as upward going


Neutrino events in standard analysis: cut

107 neutrino events

90% purity, 0.6 / day

169 days live time

0.17

(~7 reconstructed / day)

Preliminary


An alternative approach under study

Likelihood ratio method

4 discriminative variables:

• cos()

Data, MC, MC





• cos()

• of the prefit

Data, MC, MC





• cos()

• of the prefit

• Fraction of direct hits (time res<3 ns)

Data, MC, MC





• cos()

• of the prefit


• Distance covered by the last in time photon

Data, MC, MC


s(x) = pdf of MC atmospheric neutrinos

b(x) = pdf of MC atmospheric muons

y = s(x) / [ s(x) + b(x) ]




• cos()

• of the prefit


• Distance covered by the last in time photon

Data, MC, MC (x100)

y


yyData, MC, MC (x100)

y

An alternative approach under study – y correlation

Purity = 90%; Efficiency = 27%

y = s(x) / [ s(x) + b(x) ]


An alternative approach under study: neutrino events

Only + y

Under study PRELIMINARY

Data MC (Bartol flux)

MC (arXiv:0802.0562)

62 ± 8 (stat) 95 ± 3 (stat) ± 18 (theor)*

* assumed 20% uncertainty on the absolute neutrino flux

Preliminary result:

1.1 / day in data; from MC: 90% purity, 1° angular resolution

Real data: 56.4 days live time


Conclusion

Analysis of ANTARES 5 line data in progress– Present results

• Good agreement in MC and data down going flux • 107 in 169 days of 5 line data

– Still room for improvement• an alternative study leads to 1.1 / day, with 90% purity

• 10 lines data now available• 12 lines soon available!• Search for neutrino cosmic sources started…


BACKUP: track equation

Time resolution: ~2 ns

Position resolution: ~10 cm


BACKUP: reconstruction algorithm details (1)

• Step 1: linear prefit with coincidences and large amplitude hits

• Step 2: minimization of the M-estimator formula

Ai = charge, ri = time residual, fang = angular factor, K=0.05 (from MC simulation)


• Step 3: maximum likelihood fit with simple time residual PDF

P( event | track ) = prodi P( ti | tith )



)NN).t(P(N

1)t(P bkgsigsig

tot

• Step 4: maximum likelihood fit with full PDF (charge bins)

Distributions obtained from MC muons 100 GeV < E < 100 TeV



BACKUP: direct hits

Data, MC, MC


BACKUP: last in time photon

MC

Documents

CEA DSM Irfu Reconstruction and analysis of ANTARES 5 line data Niccolò Cottini on behalf of the ANTARES Collaboration XX th Rencontres de Blois 21 / 05